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javajava-8java-streamcollectors

How to create a map with Java stream API using a value outside the stream?

I want to init a Map<String, BigDecimal> and want to always put the same BigDecimal value from outside of the stream.

BigDecimal samePrice;
Set<String> set;

set.stream().collect(Collectors.toMap(Function.identity(), samePrice));

However Java complains as follows:

The method toMap(Function, Function) in the type Collectors is not applicable for the arguments (Function, BigDecimal)

Why can't I use the BigDecimal from outside? If I write:

set.stream().collect(Collectors.toMap(Function.identity(), new BigDecimal()));

it would work, but that's of course not what I want.

Solution

  • The second argument (like the first one) of toMap(keyMapper, valueMapper) is a function that takes the stream element and returns the value of the map.

    In this case, you want to ignore it so you can have:

    set.stream().collect(Collectors.toMap(Function.identity(), e -> samePrice));

    Note that your second attempt wouldn't work for the same reason.